In a color cathode ray tube, which is provided with an in-line type electron gun, the phosphor that constitutes the phosphor screen should be formed in the form of stripes. A black matrix, made of a light absorbing material such as graphite, is provided between the phosphor stripes in order to prevent the lowering of the luminance due to external light rays.
The phosphor stripes and the black matrix are typically formed through the use of a photo sensitive synthetic resin based on a photo etching method, i.e., based on exposure and development. In this situation, a shadow mask which guides the electron beams in a completed color cathode ray tube is generally used as the exposing mask. Such a conventional shadow mask is shown in FIG. 1. The shadow mask 1' is provided with numerous slit type apertures 2 having a width h.
FIG. 2 illustrates an exposing device and a method of exposing the inner face of a panel 5 using the shadow mask 1' as the exposing mask. This exposing device is constituted such that the shadow mask 1', acting as the exposing mask, is installed in the interior of the panel 5 and is coated with a photo sensitive material. A shutter having a slit 7a is moved in the direction of the shorter side of the panel 5, so that the inner face of the panel 5 will be exposed in a stepwise manner through the use of light rays radiated from a light source 6. Continuous black matrices and phosphor stripes are thereby formed.
However, the respective distances from the light source 6 to various points on the panel 5 are different from one another, and therefore, the illuminances at the different points on the panel become different from one another. As a result, the levels of exposure also become different from one another.
Accordingly, the state of the black matrices and the phosphor stripes formed on the inner face of the panel 5 may be deficient. For example, the black matrices, which are usually formed before the phosphor stripes, may create peeling at the exposed portions. Thus, as shown in FIG. 3A, in cases where the exposure is insufficient, the widths of the black matrices B may be irregularly expanded relative to the standard widths. In cases where the exposure is excessive, the widths of the black matrices B may be irregularly narrowed as shown in FIG. 3B, so that the linearity of the black matrices B is aggravated. If the linearity of the black matrices B is aggravated, then the phosphor stripes formed between the black matrices are also correspondingly aggravated.
One prior attempt to overcome such disadvantages is disclosed in Japanese Utility Model Publication No. 58-41633, in which efforts are made primarily to modify the shape of the slit of the shutter. However, this modification has not properly solved the above described problem.